Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
Horseweed (or marestail) can be challenging to control in soybeans, but planning ahead can improve horseweed control. The biggest reason for poor horseweed control is treating large plants. Horseweed over 4 to 5 inches tall are hard to control, regardless of the herbicide used. 2,4-D, Sharpen, dicamba, or Liberty can provide good to excellent control if used at the full rate and applied to small plants. The full rate of 2,4-D is 1 lb of acid (1 qt of most products) and Sharpen at 1.5 oz. Both have a waiting period between application and planting, and trying to reduce the rate of these products and reduce the waiting period often results in reduced control.
We have seen more horseweed emerging in the spring and so if a field is treated 3 to 4 weeks prior to planting there is a chance that horseweed seedlings can emerge before planting. This will either require the use of a residual herbicide with the burndown application (i.e. Valor, Authority products or metribuzin) or a second burndown application at planting; and consider paraquat for this at planting application. Applying a residual herbicide 2 to 4 weeks after planting is not the approach to take for fields with Palmer amaranth, because those fields need the residual herbicide to be applied at planting to provide adequate length of control after the soybeans are planted.
Also, using Sharpen four weeks before planting increases the options of residual herbicides at planting. The Sharpen label does not allow another group 14 herbicide (Valor, Authority product, or Reflex) within 30 days of application on coarse-textured soils with low organic matter or 14-days for all other soil types.